1. Field of the Invention
The invention relates to a sensor element which includes at least one measuring element, having piezoelectric and pyro-electric properties and is provided with measuring electrodes, where a measurement variable and a disturbance variable simultaneously act on the measuring element and the measurement signal picked up by the measuring electrodes contains a noise signal, and further including at least one compensating element with only the disturbance variable acting on the compensating element, such that a correction signal can be obtained from the compensating element, which may be used to compensate for the noise signal in the measurement signal.
2. The Prior Art
Piezoelectric sensor elements for measuring mechanical quantities, such as pressure, force, or acceleration, have a wide range of applications, the advantage of these sensors being in particular a high possibility of miniaturization and the practically movement-free measurement of the quantities cited. Different piezoelectric materials are used for different ranges of measurement, which tend to be rather expensive (e.g. GaPO4 for the high-temperature range) and/or not easily available, however.
Many piezoelectric materials (as for instance tourmaline) are well suited for sensor applications due to their high temperature stability and high piezoelectric sensitivity, but are, besides their piezoelectric properties, also pyro-electric, which is undesirable in many applications. Pressure measurements in areas with changing temperature conditions (for instance in combustion chambers of internal combustion engines) with the use of piezoelectric materials featuring pyroelectric properties as well, are accompanied by a disturbance signal which cannot be discerned from the measuring signal itself.
The reason for the disturbance signal, for instance in the case of a pressure sensor, lies in the fact that electrical charge is generated on the piezoelectric element not only by the pressure applied but also on account of temperature changes, which will result in an electrical signal having a signal component dependent on pressure change and a signal component dependent on temperature change.
In this context a force sensor for dynamic and/or quasistatic forces has become known from EP 0 055 345 A1, in which two piezoceramic platelets are located in a closed housing, one of which is connected with a coupling element transmitting the force, while the other one is a compensating element for noise signals of non-mechanical origin. The force pick-up and the compensating element have the same geometry, the difference being that force can be transmitted only to the force pick-up. The compensating element is separated from the force transmitting structure by an air gap, such that no force transmission is possible and only signals of non-mechanical origin can be registered.
An essential disadvantage of the force sensor according to the above EP 0 055 345 A1 lies in the fact that in the case of differing temperatures T1 and T2 in the force-transmitting structures, or if temperature T1 or T2 changes rapidly, different temperature gradients or temperature fields arise in the measuring element and in the compensating element and thus satisfactory compensation is not possible.
In a measurement set-up according to FIG. 3 of EP 0 055 345 A1 a temperature gradient depending on the temperatures T1 and T2 would arise in the measuring element if temperature T1 in the base or T2 in the shell would differ, while the compensation element would essentially assume the temperature T1 of the base. This effect would even be strengthened if the temperatures would change over time, as is the case for instance when pressure is measured in the combustion chambers of internal combustion engines.
It is the object of the present invention to improve a sensor element with a measuring element with both piezoelectric and pyroelectric properties in such a way that a disturbance-free measuring signal can be obtained in as simple a manner as possible.